1. Field
The present application relates to an optical receiving device and an optical transmission system, for example.
2. Description of the Related Art
Recently, as the growth of broadband services has boosted a demand for communications, the demand for increasing the capacity of a backbone transmission system of an optical communication network has been rising. Under such circumstances, a WDM transmission system of 40 Gb/s has been gradually commercialized.
There are two ways to put a WDM transmission system of 40 Gb/s into use: replacing all low speed channels of 10 Gb/s or the like by high speed channels of 40 Gb/s; and replacing some low speed channels of an existing network by high speed channels in stages to make upgrades as required. The latter way is considered practical as doing so can introduce 40 Gb/s signals at low cost.
The transmission rate of an optical fiber varies according to a wavelength of a light. As a light travels over a longer transmission distance, the light undergoes more chromatic dispersion, which makes the pulse waveform of the light less distinguishable. The chromatic dispersion, which uses the units ps/nm/km, is defined as a difference between propagation times for two monochromatic lights whose wavelengths differ by 1 nm propagated over 1 km.
When pulse broadening due to chromatic dispersion occurs in a WDM system, which is for implementing a large capacity optical transmission for a long distance, it damages the system, significantly degrading the receiving capability. This is why dispersion compensation is provided to make chromatic dispersion occurring in an optical fiber equally zero (cancel).
Control for compensating chromatic dispersion by periodically cancelling chromatic dispersion occurring on a transmission line with dispersion compensation fibers and the like installed in optical repeaters which are arranged on the transmission line is widely adopted as a chromatic dispersion compensation control method.
As a WDM transmission system, however, usually manages the dispersion compensation of a central wavelength of multiplexed wavelengths so as to make a dispersion amount zero, it cannot compensate for the dispersion of signals whose wavelengths are other than the central wavelength, leaving the dispersion accumulated at a receiver side. Accumulated chromatic dispersion which has not been compensated by dispersion compensation fibers installed on a transmission channel is called residual dispersion. A tolerance for the residual dispersion is called dispersion tolerance.
Specifically, the dispersion tolerance is a tolerance for waveform distortion derived from chromatic dispersion (a tolerance for a bit error rate). If a chromatic dispersion amount deviates from the dispersion tolerance, the receiving side cannot guarantee its ability to distinguish between the signals “0” and “1” (eye-opening degradation becomes too severe to distinguish data).
FIG. 11 and FIG. 12 illustrate dispersion tolerance characteristics. FIG. 11 illustrates a dispersion tolerance curve of 10 Gb/s. FIG. 12 illustrates a dispersion tolerance curve of 40 Gb/s. The vertical axes indicate Q penalties (dB) and the horizontal axes indicate residual dispersion (ps/nm).
A Q value is an index used for quality evaluation of optical transmission. The Q value is a value that quantifies influence on a noise in the amplitude direction. A larger Q value indicates that the transmission quality is better. The Q penalty is a value that is obtained by subtracting an actually measured Q value from an ideal Q value. A smaller Q penalty value indicates that the transmission quality is better.
Referring to FIG. 11 and FIG. 12, the Q penalty for residual dispersion rd in the case of 10 Gb/s is 1.8, whereas that in the case of 40 Gb/s is 4, for example, which means that even with the same residual dispersion amount, the transmission quality in the case of 40 Gb/s becomes significantly worse. Namely, it proves that the dispersion tolerance of 40 Gb/s is much narrower than that of 10 Gb/s.
In a low speed WDM transmission of 10 Gb/s or the like, as the dispersion tolerance, which is a tolerance for residual dispersion, is broad, the residual dispersion at an optical receiver side can be suppressed to within the tolerance with a dispersion compensation fiber, having a suitable dispersion compensation amount, installed in each relay section on the transmission line.
In contrast, in a high speed WDM transmission of 40 Gb/s, as the dispersion tolerance is significantly narrower than that of 10 Gb/s (the strength of a high speed channel of 40 Gb/s is weaker against the chromatic dispersion than a low speed channel of 10 Gb/s is), the dispersion compensation fibers arranged on the transmission line are not enough for suppressing the residual dispersion to within a desired dispersion tolerance.
Accordingly, in the case of an optical transmission over a high speed channel of 40 Gb/s or the like, the residual dispersion is compensated not only by the dispersion compensation fibers installed on a transmission line but also by a variable chromatic dispersion compensator arranged in an optical receiver.
As a conventional dispersion compensation technique, a technique for making coarse adjustment to a dispersion compensation amount by selecting a predetermined dispersion compensation fiber from a unit, which has a plurality of dispersion compensation fibers of different dispersion compensation amounts, and then making fine adjustment to the dispersion compensation amount of the signal light after the coarse adjustment with a light reflection element unit which produces group delay by changing an optical path length (for example, see Patent Document 1) is proposed. Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2003-101478 (paragraphs 0013 to 0024, FIG. 1)